Add login-required user management (admin/user roles, Config->Benutzer)

No self-signup: admins create every account in Config -> Benutzer with a
typed start password; a forced password change gates first use. Session-
cookie auth protects all /api/* routes; /api/users additionally requires
the admin role. See docs/authentication.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/config/page.tsx

@@ -0,0 +1,5 @@
+import { redirect } from "next/navigation";
+
+export default function ConfigPage() {
+  redirect("/config/benutzer");
+}